This invention pertains to telephone switching systems in general and to a digital trunk circuit for use in interfacing digital transmission lines and digital electronic telephone switching systems, in particular.
Where a digital transmission line is used to interconnect a digital switching network of a Central Office with remote equipment such as a remote line switch, a channel bank or another digital office, a carrier terminal unit (CTU) or span interface is used to provide the terminating functions for the transmission line. Presently, the primary digital carrier DS1 employed in North America consist of 24 pcm channels, 1.544 Mb/s bit rate and D2/D3 signaling format.
The receive section of a carrier terminal unit must perform the following functions:
(a) conversion of bipolar DS1 signals from the T1 line to unipolar return-to-zero and non-return-to-zero DS1 signals; PA1 (b) extract a bit clock from the return-to-zero data stream in the presence of consecutive zeroes and pattern-induced jitter; PA1 (c) provide bit and frame alignment; PA1 (d) provide frame detection including in-frame monitoring, framing search and frame alignment. PA1 a. An incoming 1.544 MHz bipolar incoming signal from a T1 line is terminated and converted from bipolar to unipolar signals. PA1 b. The bit clock of the T1 line is extracted from the incoming data stream. PA1 c. The incoming framing signal is detected within an average reframe time of 50 msec. The framing circuit detects framing on a conventional framing code or on a signal frame identification code. PA1 d. Bit and single frame timing adjustments are made to the incoming signal to align it to 2.048 MHz timing. An elastic store of one frame and one third frames is provided for single frame alignment. PA1 e. Delay variations are compensated. PA1 f. The standard incoming 1.544 MHz T1 line format of 193 bits per frame, comprising 24 8-bit voice channels and one framing bit is converted to a 2.048 MHz system format of 256 bits per frame comprising 24 10 bit voice channels, one 10-bit common signaling channel and six "don't care" bits. Each of the 10-bit voice channels includes 8 voice bits, a parity bit and a signaling bit.
It has been found particularly advantageous in one recently developed family of time-division multiplexed switching systems under development by ITT North Electric Company and called DSS to switch voice paths at a 2.048 Mb/s rate. Various aspects of the DSS family of systems have been described by N. J. Skaperda in "Generic Digital Switching System", International Switching Symposium, Vol. 1, October, 1976 by C. G. Svala in "DSS-1, A Digital Local Switching System with Remote Line Switches", proceedings of the National Telecommunications Conference, p 3915-1 1977; and by F. Fellinger in "Modular Digital Switching Network", International Communications Conference, June 4-7, 1978.
A carrier terminal unit for a system such as DSS must, in addition to providing the functions listed above, provide for converting the 1.544 Mb/s incoming DSI data stream to the 2.048 Mb/s data rate employed in DSS as well as convert the format to that employed in DSS.